1. Field of the Invention
Embodiments of the present invention are directed in general toward n and p-type materials fabricated from heteroatom-containing diamondoids, wherein the heteroatom is an electron-donating or electron-withdrawing dopant atom substitutionally positioned within the diamond lattice structure. More particularly, the present invention is directed toward semiconducting devices that may be fabricated from such n and p-type materials, including diodes, bipolar transistors, and field effect transistors.
2. State of the Art
Carbon-containing materials offer a variety of potential uses in microelectronics. A review of carbon's structure-property relationships has been presented by S. Prawer in a chapter titled “The Wonderful World of Carbon,” in Physics of Novel Materials (World Scientific, Singapore, 1999), pp. 205-234. Prawer suggests the two most important parameters that may be used to predict the properties of a carbon-containing material are, first, the ratio of sp2 to sp3 bonding in a material, and second, microstructure, including the crystallite size of the material, i.e. the size of its individual grains.
Elemental carbon has the electronic structure 1s22s22p2, where the outer shell 2s and 2p electrons have the ability to hybridize according to two different schemes. The so-called sp3 hybridization comprises four identical σ bonds arranged in a tetrahedral manner. The so-called sp2-hybridization comprises three trigonal (as well as planar) σ bonds with an unhybridized p-electron occupying a π orbital in a bond oriented perpendicular to the plane of the σ bonds. At the “extremes” of crystalline morphology are diamond and graphite. In diamond, the carbon atoms are tetrahedrally bonded with sp3-hybridization. Graphite comprises planar “sheets” of sp2-hybridized atoms, where the sheets interact weakly through perpendicularly oriented π bonds. Carbon exists in other morphologies as well, including amorphous forms called “diamond-like carbon” (DLC), and the highly symmetrical spherical and rod-shaped structures called “fullerenes” and “nanotubes,” respectively.
Diamond is an exceptional material because it scores highest (or lowest, depending on one's point of view) in a number of different categories of properties. Not only is it the hardest material known, but it has the highest thermal conductivity of any material at room temperature. It displays superb optical transparency from the infrared through the ultraviolet, has the highest refractive index of any clear material, and is an excellent electrical insulator because of its very wide bandgap. It also displays high electrical breakdown strength, and very high electron and hole mobilities.
Bulk diamond is an insulator, and does not become a semiconductor until electron-donating or electron-withdrawing impurity atoms are inserted into the diamond lattice. Such impurity atoms usually come from group IIIA or IVA (using the present day IUPAC nomenclature) of the periodic table. Prior art methods of introducing dopant atoms into diamond include in situ insertion during growth (usually growth obtained by CVD), or ex situ insertion, such as by ion implantation or by high temperature diffusion.
Ion implantation of diamond has been discussed by R. Kalish and C. Uzan-Saguy in chapter B3.1, titled “Doping of diamond using ion implantation,” in Properties, Growth and Applications of Diamond, edited by M. H. Nazaré and A. J. Neves (Inspec, London, 2001), pp. 321-330. A disadvantage of the ion implantation technique, however, is that it forces atoms into the crystal regardless of solubility or diffusivity characteristics of the atoms being inserted. This is a violent procedure that is accompanied by bond breakage and the creation of defects in the material. Unless removed by proper annealing, these defects may lead to undesirable electronic states that overshadow the desired chemical effects of the doping. However, it is more difficult to anneal diamond than it is to anneal other group IV semiconductors, such as silicon, because of the metastability of diamond bonding (sp3). Broken sp3-hybridized diamond bonding can rearrange to the more stable and electrically conductive sp2 bonding of graphite.
Some sort of annealing is necessary, on the other hand, because broken sp3 bonds and related defects caused by the implantation process can lead to electrical effects which may be mistaken for the desired chemical doping. A few of the undesirable electrical effects that may result from implanting dopants into diamond include the creation of energy levels within the bandgap which may give rise to the desired donor or acceptors states, or compensate desired doping levels; the creation of electrically conducting pathways by creating sp2 bonded states or clusters, the creation of scattering centers which may limit or reduce carrier mobilities; and finally, the creation of dopant defect complexes which may passivate the dopants.
Disadvantages of prior art methods may include the anisotropy in the electrical properties of a doped diamond material fabricated using CVD methods. For example, in the case of n-type diamond, the addition of nitrogen to CVD grown diamond results in the enhancement of the growth rate of (100) faces, with decreasing growth rates on (111) and (110) faces. In a similar manner, the probability of boron incorporation in <111> oriented films is up to an order of magnitude higher than in <100> films. This has been discussed by G. Z. Cao in chapter B3.4, titled “Nitrogen and phosphorus doping in CVD diamond,” in Properties, Growth and Applications of Diamond, edited by M. H. Nazaré and A. J. Neves (Inspec, London, 2001), pp. 345-347.
Another disadvantage of the prior art methods, also discussed by R. Kalish and C. Uzan-Saguy, is that while ion implantation is accompanied by bond breakage that has to be removed by annealing, the annealing of diamond cannot be done at sufficiently high temperatures to be effective because otherwise metastable sp3-hybridized carbon would revert to the more stable sp2-hybridized graphite. Thus, the observation of an n-type electrical semiconduction is not often observed in diamond. This is the case particularly for nitrogen as a dopant, and probably for phosphorus, lithium, and arsenic as well.
What is needed is an n or p-type diamond-like material with desired mechanical and electrical properties, where no implantation-created crystal defects are created that may lead to undesired electronic states. The n-type diamondoid-based materials of the present invention may show more favorable electrical characteristics than implanted diamond. The n and p-type diamondoid materials of the present material may be used in devices having p-n junctions, such as diodes, transistors, including single electron transistors.
A form of carbon not discussed extensively in the literature is the “diamondoid.” Diamondoids are bridged-ring cycloalkanes that comprise adamantane, diamantane, triamantane, and the tetramers, pentamers, hexamers, heptamers, octamers, nonamers, decamers, etc., of adamantane (tricyclo[3.3.1.13,7] decane), adamantane having the stoichiometric formula C10H16, in which various adamantane units are face-fused to form larger structures. These adamantane units are essentially subunits of diamondoids. The compounds have a “diamondoid” topology in that their carbon atom arrangements are superimposable on a fragment of an FCC (face centered cubic) diamond lattice. According to embodiments of the present invention, electron donating and withdrawing heteroatoms may be inserted into the diamond lattice, thereby creating an an n and p-type (respectively) material. The heteroatom is essentially an impurity atom that has been “folded into” the diamond lattice, and thus many of the disavantages of the prior art methods have been avoided.